In the use of fluid actuators employed to position parts of machinery or to employ the machine parts to resist or apply a working force in the operation of the machines to effect a given result there has existed undesirable inherent mechanical frictional forces between the moving elements of the actuators that adversely affects the desired result.
In an attempt to overcome the adverse affect of such frictional forces it has been suggested to introduce low amplitude continuous oscillations in the fluid being fed to the actuators to minimize the frictional forces from preventing control of the movement, speed and position of the moving element of the actuator and hence the part or parts of the machine with which they are associated. An example, in part, of such attempts may be found in Russian Patent Publications SU No. 617,089 dated 7/4/78 and SU No. 686,704 dated 9/25/79. In the systems of these publications the vibrations or oscillations in the vertical or horizontal directions are introduced into the fluid piston cylinder assemblies during rolling to reduce the rolling forces otherwise required and/or to better regulate the thickness of the rolled product.
These systems while representing an improvement in reducing the affects of the frictional forces, introduce another problem in that the continuous oscillating forces prevent accurate control of the desired resisting or applied force or positioning of the working element of the machine, in the above illustrated case a rolling mill. The degree of the amplitude of the oscillating forces represent a force or position greater and less than the desired force or position of the working element which in a given case can be very objectionable, for example, in a workpiece reducing element such as a platen of a press the reduced workpiece will have a varying thickness instead of a constant thickness.
The present invention relates to a method and apparatus for obtaining optimum control both in speed and accuracy of hydraulic actuators used, for example, to apply or resist a working force of machine parts or to control the distance vetween working parts of a machine. While it will be readily appreciated that the present invention will have utility in any fluid applications employing fluid actuators, for the purpose of describing the invention its use in an hydraulic press has been selected.
In this application multiple oscillation forces are introduced into the actuator means, for example several press platen piston cylinder assemblies, in opposite directions at substantially to same amplitude and in opposite phase.
More particular, instead of having an unidirectional piston cylinder assembly for controlling the force or positioning between the work effecting part two or more piston cylinder assemblies are employed, similarly positioned, in which the two assemblies or other combination of sets are arranged to produce opposing oscillations in the applied or resisting force of equal amplitude and in opposite phase and in a manner that the oscillations will not only minimize the friction forces in the piston cylinder assemblies but will not affect the critical positioning of the working parts or the application of the desired force applied by them.
In one form the invention provides a method and apparatus of operating a machine having a work effecting element, wherein the element is designed to either apply, maintain or re-establish a desired position and wherein the element is connected to and is moved by a fluid actuator means which is subject to internal undesirable affects of friction, comprising method and means for controlling the flow of fluid to the means to obtain the desired force or position of said element, imposing controlled first oscillations on the fluid in a manner to reduce the frictional forces and, substantially simultaneous which the first oscillations imposing second controlled oscillations on the fluid of substantially the same amplitude and substantially opposite in phase to the first oscillations to reduce the first oscillations disturbing the desired force or position of the element obtained through the actuator means.